How to use the molar mass calculator
- Type the formula into Chemical formula, with correct capitals: NaCl, C6H12O6, Ca(OH)2.
- Use round, square or curly brackets for groups, such as K4[Fe(CN)6]. Subscript digits such as C₆H₁₂O₆ also work.
- For a hydrate, put ·, * or . before the water: CuSO4·5H2O.
- Read the Molar mass at the top, then the table: atoms, atomic weight, mass and mass percent for each element.
What it does and when to use it
Molar mass links what you weigh on a balance to the number of particles in a reaction. You need it to turn grams into moles, make a solution of a set concentration, find percent composition, or check a percentage yield. Adding up atomic weights by hand is slow and easy to get wrong with brackets and hydrates.
Look up a single element in the periodic table. To get the mole ratios for a reaction, balance it first with the chemical equation balancer. Constants such as the Avogadro constant are in the physical constants table.
How it works
The formula is read from left to right. Each element symbol is a capital letter with an optional lowercase letter, followed by an optional count. A group in brackets is multiplied by the number after it, and groups can be nested. A number at the very start of the formula, or after a hydrate dot, multiplies the part that follows.
Molar mass = Σ (atoms of each element × its atomic weight). Mass percent = element mass ÷ molar mass × 100. The total is shown to three decimal places, keeping trailing zeros (58.440 for NaCl), but the last digit is limited by the precision of the atomic weights. Everything runs in your browser.
Worked examples
Calcium hydroxide, Ca(OH)2. 40.078 + 2 × 15.999 + 2 × 1.0080 = 74.092 g/mol. By mass it is 54.09% calcium, 43.19% oxygen and 2.72% hydrogen.
Potassium ferrocyanide, K4[Fe(CN)6]. The (CN)6 group gives 6 carbon and 6 nitrogen atoms, 17 atoms in all. The molar mass is 368.345 g/mol, and nitrogen supplies 84.042 g/mol, or 22.82%.
Plutonium dioxide, PuO2. Plutonium is taken as [244], giving 275.998 g/mol, with a warning that Pu has no standard atomic weight.
Limits and tips
- Charges and state symbols such as (aq) are not accepted here; type the neutral formula.
- Isotopes such as deuterium (D) or ¹³C are not supported.
- For a mixture, work out each compound separately.
- Round your final answer to the number of significant figures your course expects.
Frequently asked questions
Where do the atomic weights come from?
How do I enter a hydrate?
Why does it say my formula has an error?
What about elements with no stable isotopes?
Is molar mass the same as molecular weight?
Sources
- Abridged Standard Atomic Weights — IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), accessed 2026-10-08
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